1. Field of the Invention
This invention relates to the field of integrated circuit manufacturing. In particular, the invention relates to a method and a system for analyzing defects on binary intensity masks, phase-shifting masks, and next generation lithography (NGL) masks used in the manufacture of integrated circuits with job-based automation.
2. Description of Related Art
In designing an integrated circuit (IC), engineers typically rely upon computer simulation tools to help create a circuit schematic design consisting of individual devices coupled together to perform a certain function. To actually fabricate this circuit in a semiconductor substrate the circuit must be translated into a physical representation, i.e. a layout. Again, computer aided design (CAD) tools assist layout designers in the task of translating the discrete circuit elements into shapes, which will embody the devices themselves in the completed IC. These shapes make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on.
Once the layout of the circuit has been created, the next step to manufacturing the integrated circuit is to transfer the layout onto a semiconductor substrate. One way to do this is to use the process of photolithography in which the layout is first transferred onto a physical template, which is in turn used to optically project the layout onto a wafer.
In transferring the layout to the physical template, a mask (for example, a quartz plate coated with chrome) is generally created for each layer of the integrated circuit design. This is done by inputting the data representing the layout design for that layer into a device, such as an electron beam machine, which writes the integrated circuit layout pattern into the mask material. In less complicated and dense integrated circuits, each mask comprises the geometric shapes that represent the desired circuit pattern for its corresponding layer. In more complicated and dense circuits in which the size of the circuit features approach the optical limits of the lithography process, the masks may also comprise optical proximity correction (OPC) features, such as serifs, hammerheads, bias and assist bars. In other advanced circuit designs, phase-shifting masks may be used to circumvent certain basic optical limitations of the process by enhancing the contrast of the optical lithography process.
These masks are then used to optically project the layout onto a silicon wafer coated with photoresist material. For each layer of the design, a light (visible/non-visible radiation) is shone on the mask corresponding to that layer. This light passes through the clear regions of the mask, whose image exposes the underlying photoresist layer, and is blocked by the opaque regions of the mask, thereby leaving that underlying portion of the photoresist layer unexposed. (Note that the preceding example is predicated on current generation DUV lithography. In next generation lithography, e.g. x-ray lithography, the mask may operate slightly differently.) The exposed photoresist layer is then developed, typically through chemical removal of the exposed/non-exposed regions of the photoresist layer. The result is a semiconductor wafer coated with a photoresist layer exhibiting a desired pattern, which defines the geometries, features, lines and shapes of that layer. This process is then repeated for each layer of the design.
As integrated circuit designs become more complicated, it becomes increasingly important that the masks used in photolithography are accurate representations of the original design layout. Unfortunately, the electron beam and other machines used to manufacture these masks are not error-free. Thus, in the typical manufacturing process, some mask defects do occur outside the controlled process.
A defect on a mask is anything that is different from the design database and is deemed intolerable by an inspection tool or an inspection engineer. A mask can comprise a plurality of opaque areas (typically made of chrome) and a plurality of clear areas (typically made of quartz). In a bright field mask, the background is clear and the circuit pattern is defined by opaque areas. In a dark field mask, the background is opaque and the circuit pattern is defined by clear areas. Common mask defects that occur during a bright field mask manufacturing process include, for example, an isolated opaque spot defect in a clear area, an isolated clear pinhole defect in an opaque area, an edge intrusion defect in an opaque area, an edge protrusion defect in a clear area, a geometry break defect in an opaque area, and a geometry bridge defect in a clear area. Similar type defects can occur in a dark field mask manufacturing process. Defects may also occur in the OPC features provided on the chip.
After designing an integrated circuit and creating a data file, the mask design data is provided to a device such as an electron beam or laser writing machine and a mask is manufactured. The mask is then inspected for defects. In this inspection, the surface of the mask can be scanned with a high resolution microscope (e.g. optical, scanning electron, focus ion beam, atomic force, and near-field optical microscopes), which captures images of the mask.
These mask images can then be observed off-line by an engineer or on-line by a mask fabrication worker to identify defects on the physical mask. Then, a decision is made whether the inspected mask is good enough for use in the photolithography process. This decision can be made off-line by a skilled inspection engineer or on-line by a fabrication worker, possibly with the aid of inspection software. If there are no defects, or defects are discovered but determined to be within tolerances set by the manufacturer or user, then the mask passes inspection and can be used to expose a wafer. If defects are discovered that fall outside tolerances, then the mask fails inspection and a decision is made as to whether the mask can be cleaned and/or repaired to correct the defects, or whether the defects are so severe that a new mask must be manufactured. This process is continued until a manufactured mask passes inspection.
In one embodiment, the mask can be further inspected to ensure that the mask will produce the desired image on a photoresist after a wafer is exposed to light through the mask. Frequently, this inspection includes exposing and processing a wafer using the inspected mask. Then, a decision is made as to whether there are any defects on the processed wafer and whether the defects fall within tolerances. If discovered defects are substantial, then, as before, a decision is made whether the defects can be repaired or whether a new mask must be produced. This process is continued until a mask is manufactured that will produce the desired wafer patterns and that will pass the wafer level inspection, thereby ending inspection. This mask is then used in the photolithography process to expose the corresponding layer in the overall manufacturing process.
The goal of defect inspection is to correctly identify a defect to avoid a failed wafer processing. However, not all mask defects are important with respect to the desired result, i.e. an accurate representation of the original design layout on the photoresist material or etched into silicon. Specifically, not all mask defects will “print.” Loosely speaking, the printability of a defect is how a defect would impact the outcome of a given photolithography and/or wafer processing approach, including etching, implantation, etc. Because the printability of a defect is mainly associated with the stepper exposure conditions, a defect can be “not printable” for a particular set of stepper exposure conditions and “printable” under a different set of stepper exposure conditions. These conditions for optical photolithography can include, for example, defect size, wavelength, numerical aperture, coherence factor, illumination mode, exposure time, exposure focus/defocus, defect location, surrounding features, and the reflection/transmission characteristics of the defect.
Accordingly, in any mask inspection system, the important decision to be made is whether a given defect will “print” on the underlying photoresist in a photolithography process under specified conditions. If a mask defect does not print or have other effects on the photolithography process (such as unacceptably narrowing the process window), then the mask with the defect can still be used to provide acceptable results. Therefore, one can avoid the expense in time and money of repairing and/or replacing masks whose defects do not print. What is desired then, is a method and system for quickly and accurately analyzing defects on the masks used in the photolithography process.